A mouse aging contract was initiated with Jackson Laboratories to provide side-by-side cohorts of 2 and 10-20 month old mice. Since the aged mice are retired breeders, young mice are put through a round of birth and weaning. Six metastasis experiments have been conducted. Four, with lung metastases as the primary endpoint, showed no effect of host age. Two, with brain metastasis as the primary endpoint, showed increased brain metastases in the young mice. Current efforts are examining (1) other brain metastasis models, (2) the role of the young immune system in producing increased metastases, (3) proteomic differences in the metastatic microenvironment of young and aged brains.